1. Field of the Invention
This invention relates to a digital-analog converter which converts digital signals into analog signals. Especially, it relates to the digital-analog converter of the resistor string type.
2. Description of the Prior Art
Digital-analog converters convert digital signals to analog signals. In the prior art these converters are generally of the resistor string and R-2R types. The resistor string type has resistors connected in series and analog output voltages are at connecting points of the resistors, while the R-2R type has resistors, with unit resistor values of R and 2R, connected to each other in a ladder-like form.
Digital-analog converters of the resistor string type are widely used because they have a simple construction, provide comparatively high precision, guarantee monotonicity, and they do not produce any bit-errors.
FIG. 4 shows the conventional circuit components of a 4-bit digital-analog converter of the resistor string type.
This digital-analog converter is equipped with a resistor string 401 which consists of unit resistors R101 through R116 having a resistor value of R and connected with each other in series. Connected to the both ends of this resistor string 401 are the first reference voltage terminal 411 and the second reference voltage terminal 412, respectively.
Analog switches SW101 through SW116 are provided between each connecting point of the unit resistors R101 through R116 and an analog signal output terminal 413 which produces converted analog signals. These analog switches SW101 through SW116 are turned on and off by the signals into which a control-signal generating circuit 403 has decoded bits D3, D2, D1 and DO given at the respective digital input signal terminals 431 through 434. However, control-signal terminals between the analog switches SW101 through SW116 and the control-signal generating circuit 403 are not shown in FIG. 4. In this example of prior art, the well-known 4-16 decoder circuit is used for the control-signal generating circuit 403.
The operation of the conventional digital-analog converter of the resistor string type will be described below with reference to FIG. 4.
In the drawing, the voltage given at the first reference voltage terminal 411 is VR1, the voltage given at the second reference voltage terminal 412 is VR2, the analog output voltage at the analog signal output terminal 413 is V0, the signal bit D3 given to the digital input signal terminal 431 is the most significant bit (MSB), bit D2 given at the terminal 432 is the second bit (2SB), bit D1 given at the terminal 433 is the third bit (3SB), and bit D0 given at the terminal 434 is the least significant bit (LSB).
The difference of the voltage between VR1 and VR2, i.e. (VR1-VR2) is divided, and analog output voltage shown as V0=VR2+i/16.times.(VR1-VR2); i=1 to 15 is obtained at each connecting point of the unit resistors R101 through R116, wherein the voltages VR1 and VR2 are given to the first and second reference voltage terminals 411 and 412, respectively by the unit resistors R101 through R116 of the resistor string 401. The analog output voltage V0 corresponding to the digital signals D3 through D0 is provided to the analog signal output terminal 413 by turning on and off the switches SW101 through SW116 according to the combination of the digital signals D3 through D0.
Table 1-1 through 1-2 shows ON's and OFF's of the switches SW101 through SW116 according to the combination of bits D3 through D0 of each digital input signal and their corresponding analog output voltages. It is noted that an ON and OFF show the conductive and non-conductive conditions of a switch, respectively.
TABLE 1-1 __________________________________________________________________________ Digital input signal Conditions of a switch D3 D2 D1 D0 SW101-SW116 __________________________________________________________________________ 0 0 0 0 SW101-115 = OFF SW116 = ON 0 0 0 1 SW101-114 = OFF SW115 = ON SW116 = OFF 0 0 1 0 SW101-113 = OFF SW114 = ON SW115-116 = OFF 0 0 1 1 SW101-112 = OFF SW113 = ON SW114-116 = OFF 0 1 0 0 SW101-111 = OFF SW112 = ON SW113-116 = OFF 0 1 0 1 SW101-110 = OFF SW111 = ON SW112-116 = OFF 0 1 1 0 SW101-109 = OFF SW110 = ON SW111-116 = OFF 0 1 1 1 SW101-108 = OFF SW109 = ON SW110-116 = OFF 1 0 0 0 SW101-107 = OFF SW108 = ON SW109-116 = OFF 1 0 0 1 SW101-106 = OFF SW107 = ON SW108-116 = OFF 1 0 1 0 SW101-105 = OFF SW106 = ON SW107-116 = OFF 1 0 1 1 SW101-104 = OFF SW105 = ON SW106-116 = OFF 1 1 0 0 SW101-103 = OFF SW104 = ON SW105-116 = OFF 1 1 0 1 SW101-102 = OFF SW103 = ON SW104-116 = OFF 1 1 1 0 SW101 = OFF SW102 = ON SW103-116 = OFF 1 1 1 1 SW101 = ON SW102-116 = OFF __________________________________________________________________________
TABLE 1-2 ______________________________________ Digital input signal D3 D2 D1 D0 Analog output voltages ______________________________________ 0 0 0 0 VR2 + 0/16 .times. (VR1-VR2) 0 0 0 1 VR2 + 1/16 .times. (VR1-VR2) 0 0 1 0 VR2 + 2/16 .times. (VR1-VR2) 0 0 1 1 VR2 + 3/16 .times. (VR1-VR2) 0 1 0 0 VR2 + 4/16 .times. (VR1-VR2) 0 1 0 1 VR2 + 5/16 .times. (VR1-VR2) 0 1 1 0 VR2 + 6/16 .times. (VR1-VR2) 0 1 1 1 VR2 + 7/16 .times. (VR1-VR2) 1 0 0 0 VR2 + 8/16 .times. (VR1-VR2) 1 0 0 1 VR2 + 9/16 .times. (VR1-VR2) 1 0 1 0 VR2 + 10/16 .times. (VR1-VR2) 1 0 1 1 VR2 + 11/16 .times. (VR1-VR2) 1 1 0 0 VR2 + 12/16 .times. (VR1-VR2) 1 1 0 1 VR2 + 13/16 .times. (VR1-VR2) 1 1 1 0 VR2 + 14/16 .times. (VR1-VR2) 1 1 1 1 VR2 + 15/16 .times. (VR1-VR2) ______________________________________
In such a digital-analog converter of the resistor string type, the conversion of digital signals of n bits requires unit resistors in the quantity of 2 to the n-th power. For instance, in case of 8 bits, many resistors are required, i.e. 2 to the 8th power, or 256 resistors. In addition, the same quantity of switches is also required, i.e. 2 to the n-th power. Accordingly, when a digital-analog converter is formed in an integrated circuit, for example, a disadvantage is that unit resistors and switches require a large space.
Furthermore, it is very difficult to provide unit resistors that have the same resistance value in a very large space on an integrated circuit due to the dispersion in the production and defects of IC substrate. This also provides poor conversion characteristics.
Another disadvantage is that testing of a digital-analog converter requires a very long process because the resistance value of each unit resistor does affect the output characteristics directly in the resistor string type. In case of 8 bits, it is necessary to measure the digital input signal 256 times from the zero scale to the full scale. The zero scale has 0's in all the bits from the MSB to the LSB, and the full scale has 1's in all the bits.